Generally, medical X-ray detectors employing a scintillating phosphor screen to absorb X-rays and produce light suffer the loss of spatial resolution due to lateral light diffusion in the phosphor screen. To reduce lateral light diffusion and maintain acceptable spatial resolution, the phosphor screens must be made sufficiently thin. The spatial resolution and X-ray detection ability of an imaging apparatus are often characterized by the modulation transfer function (MTF) and X-ray absorption efficiency, respectively. Thin phosphor screens produce better MTF at the expense of reduced X-ray absorption. Usually, the coating density and the thickness of the phosphor screen are used in the design tradeoff between spatial resolution and X-ray absorption efficiency.
In order to improve X-ray absorption and maintain spatial resolution, the use of dual screens is known in conjunction with digital computed radiography (CR) to improve the X-ray absorption efficiency. In such CR apparatuses, a storage phosphor screen is used in place of the prompt emitting phosphor screen employed in traditional screen-film apparatus. No film is needed for CR. Upon X-ray exposure, the storage phosphor screen stores a latent image in the form of trapped charge that is subsequently read out, typically by a scanning laser beam, to produce a digital radiographic image.
Recently, digital flat panel imaging detector arrays based upon active matrix thin film electronics have shown promise for applications such as diagnostic radiology and digital mammography. There are two types of X-ray energy conversion methods used in digital radiography (DR), namely: the direct method and the indirect method. In the direct method, the X-rays absorbed in a photoconductor are directly transduced into a charge signal, stored on the pixel electrodes on an active matrix array (AMA) and read out using thin film transistors (TFTs) to produce a digital image. Amorphous selenium (a-Se) is typically used as the photoconductor. No phosphor screen is required for the direct method. In the indirect method, a phosphor screen is used to absorb X-rays and the light photons emitted by the phosphor screen are detected by an AMA with a single photodiode (PD) and a TFT switch at each pixel. The photodiode absorbs the light given off by the phosphor in proportion to the X-ray energy absorbed. The stored charge is then read out, like the direct method, using the TFT switch. Several types of imaging arrays based on thin-film-transistors can be used for image sensing. These include hydrogenated amorphous-silicon (a-Si:H) photodetectors with amorphous-silicon TFT switches, amorphous silicon photodetectors with low-temperature-polysilicon (LTPS), and organic photodetectors with organic TFT (OTFT) switches.
FIG. 1 shows a block diagram of circuitry for a typical type of known flat panel imager 10, which includes a sensor array 12. The a-Si based sensor array includes m data lines 14 and n row select or gate lines 16. Each pixel comprises an a-Si photodiode 18 connected to a TFT 20. Each photodiode 18 is connected to a common bias line 22 and a drain 24 of its associated TFT. Gate lines 16 are connected to gate drivers 26. Bias lines 22 carry bias voltages applied to photodiodes 18 and TFTs 20. TFTs 20 are controlled by their associated gate lines 26 and when addressed, transfer stored charge onto data lines 14. During readout, a gate line is turned on for a finite time (approximately 10 to 100 μs), allowing sufficient time for TFTs 20 on that row to transfer their pixel charges to all the m data lines. Data lines 14 are connected to charge amplifiers 28, which operate in parallel. In general, charge amplifiers 28 are divided into a number of groups, with each group typically having 32, 64, or 128 charge amplifiers. The associated charge amplifiers in each group detect the image signals, and clock the signals onto multiplexer 30, whence they are multiplexed and subsequently digitized by an analog to digital converter 32. The digital image data are then transferred over a coupling to memory. In some designs, a correlated double sampling (CDS) circuit 34 may be disposed between each charge amplifier 28 and multiplexer 30 to reduce electronic noise. Gate lines 16 are turned on in sequence, requiring approximately a few seconds for an entire frame to be scanned. Additional image correction and image processing are performed by a computer 36 and the resulting image is displayed on a monitor 38 or printed by a printer 40.
FIG. 2 shows a cross-section (not to scale) of a single, typical type of known imaging pixel 50 such as is used in conventional a-Si based flat panel imagers in which the image sensing element is a PIN photodiode. Each imaging pixel 50 has a PIN photodiode 52 and a TFT switch 54 formed on a substrate 56. A layer of X-ray converter (e.g., a scintillating phosphor screen 58) is coupled to the photodiode-TFT array. TFT switch 54 comprises the following layers: a first layer of metal 60 forming a TFT gate electrode and row select lines, an insulator layer 62 forming a gate dielectric for the TFT, an intrinsic amorphous silicon layer 64 forming a channel region for the TFT, amorphous silicon making up an n-type dopant layer 66 forming the source and the drain for the TFT, a second layer of metal 68 forming TFT source and drain contacts and data lines, and an insulator layer 70. PIN photodiode 52 includes the following layers: a third layer of metal 72 forming a back contact of the PIN photodiode and an interconnect between the TFT and the PIN photodiode, an amorphous silicon film 74 containing a p-type dopant, an intrinsic amorphous silicon film 76, an amorphous silicon film 78 containing a p-type dopant, a transparent contact electrode 80 such as indium-tin oxide, an insulator layer 81, and a fourth layer of metal 82 forming a topside contact of the PIN photodiode. An X-ray photon path 84 and visible light photon paths 86 are also shown in FIG. 2. When a single X-ray is absorbed by the screen 58, a large number of light photons are emitted isotropically. Only a fraction of the emitted light reaches the photodiode and is detected. The operation of such an a-Si based pixel with a-Si PIN electrodes is understood by those skilled in the art.
FIG. 3 shows a cross-section of two adjacent pixels 90 of another type of known image sensor array 92. In this architecture a photodiode 94 is vertically integrated above a TFT switch 96 instead of the side-by-side arrangement shown in FIG. 2. The vertically integrated sensor array is comprised of a substrate 98, a first layer of metal 100 forming the gate electrode of the TFT and the row select lines, an insulator layer 102 forming the gate dielectric of the TFT, an intrinsic (that is, not doped) amorphous silicon layer 104 forming the channel of the TFT, an n-doped amorphous silicon film 106 forming source and drain regions of the TFT and a second layer of metal 108 patterned to form source and drain contacts and the data lines. An insulator layer 110 is used to separate a TFT plane 112 from a PIN photodiode plane 114. The PIN photodiode comprises a third layer of metal 116 forming a back contact electrode, sequential deposition of an n-doped layer 118, an intrinsic amorphous silicon layer 120 and a p-doped layer 122 of amorphous silicon, followed by a transparent contact electrode 124. The photodiode layers are patterned to form individual photosensitive elements. An insulating layer 126, and a fifth layer of metal 128 forming a bias line complete the pixel. The vertically-integrated configuration offers improved photosensitivity as compared to the side-by-side configuration, due to a higher fraction of photosensitive area to pixel area (termed fill-factor).
FIG. 4 shows a cross-section (not to scale) of yet another type of known imaging pixel 140 in a prior art a-Si based flat panel imager in which the image sensing element is a metal-insulating-semiconductor (MIS) photosensor 142. Each imaging pixel 140 includes MIS photosensor 142 and a TFT switch 144 formed on a substrate 146. TFT switch 144 includes the following layers: a first layer of metal 148 forming TFT gate electrode and row select lines, an insulator layer 150 forming a gate dielectric for the TFT, an intrinsic amorphous silicon layer 152 forming a channel region for the TFT, amorphous silicon containing an n-type dopant layer 154 forming the source and drain for the TFT, an insulator layer 156 and a second layer of metal 158 forming TFT source and drain contacts and data lines. MIS photodiode 142 includes the following layers: first layer of metal 148 forming the gate electrode for the MIS photosensor, insulator layer 150 forming the gate dielectric, amorphous silicon film layer 152 forming the channel region, amorphous silicon film 154 forming the drain, a transparent electrode 160 in contact with n-type layer 154, insulator layer 156 and second layer of metal 158 forming a topside contact. The operation of such an a-Si based indirect flat panel imager with MIS photo-sensors is known to those skilled in the art.
It will be recognized by those skilled in the art that other types of photosensors, such as continuous PIN photodiodes, continuous MIS photosensors, phototransistors, and photoconductors can be realized in a variety of materials, including amorphous, polycrystalline or single-crystal silicon and non-silicon semiconductors. It will also be recognized by those skilled in the art that other pixel circuits, such as three-transistor active pixel, four-transistor active pixel and shared transistor active pixel circuits, can be used to form a radiographic imaging array.
It will be recognized by those skilled in the art that many other architectures for readout arrays are commonly used. It will also be recognized by those skilled in the art that semiconductor materials other than amorphous silicon, such as polycrystalline silicon, organic semiconductors, and various alloy semiconductors such as zinc oxide can be used for the backplane array and the sensing array. Recently, thin film transistor arrays have been fabricated on flexible substrates (made of plastics, metal foils, or other suitable organic and inorganic materials) rather than on the conventional non-flexible and brittle glass substrate. The TFT arrays on flexible substrates have been combined with liquid crystals for flexible transmissive and reflective displays, with organic light emitting devices for emissive displays, and with photosensors for visible light imaging and radiographic imaging applications.
Reference is made to commonly assigned, copending U.S. patent applications (a) Ser. No. 11/951,483 filed Dec. 6, 2007 by VanMetter et al. entitled CARDIAC GATING FOR DUAL-ENERGY IMAGING; (b) Ser. No. 60/889,356 filed Feb. 6, 2007 by VanMetter entitled DUAL ENERGY DECOMPOSITION RENORMALIZATION; and (c) Ser. No. 60/896,322 filed Mar. 22, 2007 by Dhanantwari et al. entitled REGISTRATION METHOD FOR PROJECTIONS IN DUAL ENERGY. These applications concern inventions regarding another imaging technique, known as dual energy subtraction imaging, that can be used to reduce the impact of anatomic background on disease detection in digital chest radiography and angiography. This technique is based on the different energy-dependent absorption characteristics of bone and soft tissue. In general, two raw digital images are produced. One is a low-energy and high-contrast image, and the other is a high-energy and low-contrast image. By taking nonlinear combinations of these two images, pure bone and soft-tissue images can be obtained. This imaging technique would improve diagnosis of pathology and delineation of anatomy using images.
In U.S. patent application Ser. No. 11/487,539, several dual digital radiography arrays, each imaging a respective phosphor screen, are disclosed. In one embodiment X-rays are directed through an object to a digital radiography imager to form an image. The digital radiography imager uses two flat panels (a front panel and a back panel) to capture and process X-rays in order to form an image. Preferably, the thickness of the scintillating phosphor layer of the back panel is greater than or equal to the thickness of the scintillating phosphor layer of the front panel. A filter is placed between the front panel and the back panel to minimize the crossover of light emitted in one panel to the other panel. Each panel has a first array of signal sensing elements and readout devices and a second array of signal sensing elements and readout devices. In addition, a first passivation layer is disposed on the first array of signal sensing elements and readout devices, and a second passivation layer is disposed on the second array of signal sensing elements and readout devices. The front panel and back panel are exposed to X-rays simultaneously. The first scintillating phosphor layer is responsive to X-rays passing through the object and produces light which illuminates the signal sensing elements of the first array of signal sensing elements and readout devices to provide signals representing a first X-ray image. The second scintillating phosphor layer is responsive to X-rays passing through the object and the front panel to produce light which illuminates the signal elements of the second array of signal sensing elements and readout devices to provide signals representing a second X-ray image. The signals of the first and second X-ray images can be combined to produce a composite X-ray image of a higher quality.
In another embodiment disclosed in U.S. patent application Ser. No. 11/487,539, separate flat-panel imagers are fabricated on each of the two sides of a substrate to form a digital radiographic imaging array. The first imager is primarily sensitive to the light from a first phosphor screen, which is placed in proximity to the first imager. The second imager is primarily sensitive to the light emitted from a second phosphor screen, which is placed in proximity to the second imager. Instead of using two front and back panels to capture the radiographic images, the digital radiography imager uses a single substrate having a first phosphor layer coated on the front side of the substrate and a second phosphor layer coated on the back side of the substrate. In one aspect of this embodiment, the second scintillating phosphor layer can have a thickness which is greater than or equal to the thickness of first scintillating phosphor layer. An NIP photodiode is used on each side of the substrate. A light blocking layer or crossover reducing layer is coated on each side of the substrate to minimize the crossover of light emitted in phosphor screen on one side of the substrate to the photodiode on the other side of the substrate. The first and second scintillating phosphor layers are exposed to X-rays simultaneously and the photodiode on the front and back sides of substrate detect the front and back image respectively.
A need has existed for extending the application of dual scintillating screens (scintillating phosphor layers) to an indirect digital radiography (DR) apparatus. Moreover, there exists a need for extending the application of dual scintillating screens in an indirect DR apparatus for single-exposure dual energy subtraction imaging.